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Subject[PATCHv3 1/3] fs: add O_BENEATH flag to openat(2)
Date
Add a new O_BENEATH flag for openat(2) which restricts the
provided path, rejecting (with -EPERM) paths that are not beneath
the provided dfd. In particular, reject:
- paths that contain .. components
- paths that begin with /
- symlinks that have paths as above.

Also disallow use of nd_jump_link() for following symlinks without
path expansion, when O_BENEATH is set.

Signed-off-by: David Drysdale <drysdale@google.com>
---
arch/alpha/include/uapi/asm/fcntl.h | 1 +
arch/parisc/include/uapi/asm/fcntl.h | 1 +
arch/sparc/include/uapi/asm/fcntl.h | 1 +
fs/fcntl.c | 4 ++--
fs/namei.c | 21 ++++++++++++++++++---
fs/open.c | 4 +++-
fs/proc/base.c | 4 +++-
fs/proc/namespaces.c | 8 ++++++--
include/linux/namei.h | 3 ++-
include/uapi/asm-generic/fcntl.h | 4 ++++
10 files changed, 41 insertions(+), 10 deletions(-)

diff --git a/arch/alpha/include/uapi/asm/fcntl.h b/arch/alpha/include/uapi/asm/fcntl.h
index 09f49a6b87d1..76a87038d2c1 100644
--- a/arch/alpha/include/uapi/asm/fcntl.h
+++ b/arch/alpha/include/uapi/asm/fcntl.h
@@ -33,6 +33,7 @@

#define O_PATH 040000000
#define __O_TMPFILE 0100000000
+#define O_BENEATH 0200000000 /* no / or .. in openat path */

#define F_GETLK 7
#define F_SETLK 8
diff --git a/arch/parisc/include/uapi/asm/fcntl.h b/arch/parisc/include/uapi/asm/fcntl.h
index 34a46cbc76ed..3adadf72f929 100644
--- a/arch/parisc/include/uapi/asm/fcntl.h
+++ b/arch/parisc/include/uapi/asm/fcntl.h
@@ -21,6 +21,7 @@

#define O_PATH 020000000
#define __O_TMPFILE 040000000
+#define O_BENEATH 080000000 /* no / or .. in openat path */

#define F_GETLK64 8
#define F_SETLK64 9
diff --git a/arch/sparc/include/uapi/asm/fcntl.h b/arch/sparc/include/uapi/asm/fcntl.h
index 7e8ace5bf760..ea38f0bd6cec 100644
--- a/arch/sparc/include/uapi/asm/fcntl.h
+++ b/arch/sparc/include/uapi/asm/fcntl.h
@@ -36,6 +36,7 @@

#define O_PATH 0x1000000
#define __O_TMPFILE 0x2000000
+#define O_BENEATH 0x4000000 /* no / or .. in openat path */

#define F_GETOWN 5 /* for sockets. */
#define F_SETOWN 6 /* for sockets. */
diff --git a/fs/fcntl.c b/fs/fcntl.c
index ee85cd4e136a..3169693e9390 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -740,7 +740,7 @@ static int __init fcntl_init(void)
* Exceptions: O_NONBLOCK is a two bit define on parisc; O_NDELAY
* is defined as O_NONBLOCK on some platforms and not on others.
*/
- BUILD_BUG_ON(21 - 1 /* for O_RDONLY being 0 */ != HWEIGHT32(
+ BUILD_BUG_ON(22 - 1 /* for O_RDONLY being 0 */ != HWEIGHT32(
O_RDONLY | O_WRONLY | O_RDWR |
O_CREAT | O_EXCL | O_NOCTTY |
O_TRUNC | O_APPEND | /* O_NONBLOCK | */
@@ -748,7 +748,7 @@ static int __init fcntl_init(void)
O_DIRECT | O_LARGEFILE | O_DIRECTORY |
O_NOFOLLOW | O_NOATIME | O_CLOEXEC |
__FMODE_EXEC | O_PATH | __O_TMPFILE |
- __FMODE_NONOTIFY
+ __FMODE_NONOTIFY| O_BENEATH
));

fasync_cache = kmem_cache_create("fasync_cache",
diff --git a/fs/namei.c b/fs/namei.c
index c83145af4bfc..67e9a8e27329 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -705,13 +705,16 @@ static inline void path_to_nameidata(const struct path *path,
* Helper to directly jump to a known parsed path from ->follow_link,
* caller must have taken a reference to path beforehand.
*/
-void nd_jump_link(struct nameidata *nd, struct path *path)
+int nd_jump_link(struct nameidata *nd, struct path *path)
{
+ if (nd->flags & LOOKUP_BENEATH)
+ return -EPERM;
path_put(&nd->path);

nd->path = *path;
nd->inode = nd->path.dentry->d_inode;
nd->flags |= LOOKUP_JUMPED;
+ return 0;
}

void nd_set_link(struct nameidata *nd, char *path)
@@ -1775,9 +1778,14 @@ static int link_path_walk(const char *name, struct nameidata *nd)
{
struct path next;
int err;
-
- while (*name=='/')
+
+ while (*name == '/') {
+ if (nd->flags & LOOKUP_BENEATH) {
+ err = -EPERM;
+ goto exit;
+ }
name++;
+ }
if (!*name)
return 0;

@@ -1796,6 +1804,10 @@ static int link_path_walk(const char *name, struct nameidata *nd)
if (name[0] == '.') switch (hashlen_len(hash_len)) {
case 2:
if (name[1] == '.') {
+ if (nd->flags & LOOKUP_BENEATH) {
+ err = -EPERM;
+ goto exit;
+ }
type = LAST_DOTDOT;
nd->flags |= LOOKUP_JUMPED;
}
@@ -1847,6 +1859,7 @@ static int link_path_walk(const char *name, struct nameidata *nd)
break;
}
}
+exit:
terminate_walk(nd);
return err;
}
@@ -1886,6 +1899,8 @@ static int path_init(int dfd, const char *name, unsigned int flags,

nd->m_seq = read_seqbegin(&mount_lock);
if (*name=='/') {
+ if (flags & LOOKUP_BENEATH)
+ return -EPERM;
if (flags & LOOKUP_RCU) {
rcu_read_lock();
nd->seq = set_root_rcu(nd);
diff --git a/fs/open.c b/fs/open.c
index 33f9cbf2610b..16e06610ddbb 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -902,7 +902,7 @@ static inline int build_open_flags(int flags, umode_t mode, struct open_flags *o
* If we have O_PATH in the open flag. Then we
* cannot have anything other than the below set of flags
*/
- flags &= O_DIRECTORY | O_NOFOLLOW | O_PATH;
+ flags &= O_DIRECTORY | O_NOFOLLOW | O_PATH | O_BENEATH;
acc_mode = 0;
} else {
acc_mode = MAY_OPEN | ACC_MODE(flags);
@@ -933,6 +933,8 @@ static inline int build_open_flags(int flags, umode_t mode, struct open_flags *o
lookup_flags |= LOOKUP_DIRECTORY;
if (!(flags & O_NOFOLLOW))
lookup_flags |= LOOKUP_FOLLOW;
+ if (flags & O_BENEATH)
+ lookup_flags |= LOOKUP_BENEATH;
op->lookup_flags = lookup_flags;
return 0;
}
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 3f3d7aeb0712..eb8a9926ebb1 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1385,7 +1385,9 @@ static void *proc_pid_follow_link(struct dentry *dentry, struct nameidata *nd)
if (error)
goto out;

- nd_jump_link(nd, &path);
+ error = nd_jump_link(nd, &path);
+ if (error)
+ goto out;
return NULL;
out:
return ERR_PTR(error);
diff --git a/fs/proc/namespaces.c b/fs/proc/namespaces.c
index c9eac4563fa8..8351534dc43b 100644
--- a/fs/proc/namespaces.c
+++ b/fs/proc/namespaces.c
@@ -36,6 +36,7 @@ static void *proc_ns_follow_link(struct dentry *dentry, struct nameidata *nd)
const struct proc_ns_operations *ns_ops = PROC_I(inode)->ns_ops;
struct task_struct *task;
struct path ns_path;
+ int err;
void *error = ERR_PTR(-EACCES);

task = get_proc_task(inode);
@@ -44,8 +45,11 @@ static void *proc_ns_follow_link(struct dentry *dentry, struct nameidata *nd)

if (ptrace_may_access(task, PTRACE_MODE_READ)) {
error = ns_get_path(&ns_path, task, ns_ops);
- if (!error)
- nd_jump_link(nd, &ns_path);
+ if (!error) {
+ err = nd_jump_link(nd, &ns_path);
+ if (err)
+ error = ERR_PTR(err);
+ }
}
put_task_struct(task);
return error;
diff --git a/include/linux/namei.h b/include/linux/namei.h
index c8990779f0c3..bd656840a271 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -28,6 +28,7 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND};
#define LOOKUP_FOLLOW 0x0001
#define LOOKUP_DIRECTORY 0x0002
#define LOOKUP_AUTOMOUNT 0x0004
+#define LOOKUP_BENEATH 0x0008

#define LOOKUP_PARENT 0x0010
#define LOOKUP_REVAL 0x0020
@@ -70,7 +71,7 @@ extern int follow_up(struct path *);
extern struct dentry *lock_rename(struct dentry *, struct dentry *);
extern void unlock_rename(struct dentry *, struct dentry *);

-extern void nd_jump_link(struct nameidata *nd, struct path *path);
+extern int nd_jump_link(struct nameidata *nd, struct path *path);
extern void nd_set_link(struct nameidata *nd, char *path);
extern char *nd_get_link(struct nameidata *nd);

diff --git a/include/uapi/asm-generic/fcntl.h b/include/uapi/asm-generic/fcntl.h
index e063effe0cc1..4542bc6a2950 100644
--- a/include/uapi/asm-generic/fcntl.h
+++ b/include/uapi/asm-generic/fcntl.h
@@ -92,6 +92,10 @@
#define O_TMPFILE (__O_TMPFILE | O_DIRECTORY)
#define O_TMPFILE_MASK (__O_TMPFILE | O_DIRECTORY | O_CREAT)

+#ifndef O_BENEATH
+#define O_BENEATH 040000000 /* no / or .. in openat path */
+#endif
+
#ifndef O_NDELAY
#define O_NDELAY O_NONBLOCK
#endif
--
2.2.0.rc0.207.ga3a616c

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